C processes in Linux

fork() is one of several functions to "spawn" a process

Use of fork

fork() creates a new process by duplicating the calling process. The new process is referred to as the child process. The calling process is referred to as the parent process.

• PID - Process ID - the ID of a running process
• PPID - Parent Process ID - default the ID of the process that created this process or 1 if the parent died.

 1 #include <stdio.h>
 2 #include <sys/types.h>
 3 #include <unistd.h>
 4 
 5 
 6 int main(void) {
 7         printf("I am parent - my PPID is %d and my PID is %d\n", getppid(), getpid() );
 8 
 9         if (fork() == 0) {
10                 printf("I am child  - my PPID is %d and my PID is %d\n", getppid(), getpid() );
11                 printf("Child  dying\n");
12                 return(0);
13         }
14         printf("parent dying\n");
15         return(0);
16 }

Example of creating a daemon in Linux

Example of creating a daemon in Linux: (See systemd debian for systemd daemons)

• setsid() - damon will be the session leader. Sets Session ID (SID) and Group ID (GID) to Process ID (PID)
• Reopening stdin, stdout and stderr to /dev/null detaches the process from the controlling terminal and Parent PID is set to 1 (init or systemd - depending on Linux version/distribution)

 1 #include <stdio.h>
 2 #include <unistd.h>
 3 #include <stdlib.h>
 4 #include <syslog.h>
 5 void  daemon_create(void);
 6 
 7 int main(void) {
 8         printf("Starting my_daemon\n");
 9         openlog("my_daemon", LOG_INFO, LOG_DAEMON);
10         daemon_create();
11         for (int i=0; i < 100; i++) {
12                 syslog(LOG_INFO,"Countet to %d", i);
13                 sleep(2);
14         }
15         return(0);
16 }
17 
18 void  daemon_create(void) {
19     pid_t pid;
20 
21     pid = fork();
22     if(pid < 0){
23         syslog(LOG_ERR, "Error in fork: %m");
24         exit(1);
25     }
26     if(pid > 0) {
27         syslog(LOG_INFO, "Parent dying");
28         exit(0); // Parent die
29     }
30     if(setsid() < 0){ // Make child session leader
31         syslog(LOG_ERR, "Error in setsid: %m");
32         exit(2);
33         }
34         if (freopen("/dev/null", "r", stdin) == NULL
35         || freopen("/dev/null", "w", stdout) == NULL
36         || freopen("/dev/null", "w", stderr) == NULL ) {
37                 syslog(LOG_ERR, "When creating daemon freopen from STDIN/OUT/ERR failed: %m");
38                 exit(3);
39    }
40 }

Checking the daemon

• Daemon detached from TTY - PPID is 1
• Session and session group leader - SID and PGID same as PID

$ ps xo uname,pid,ppid,pgid,sid,tty,comm | grep -P '(my_daemon|COMMAND)'
USER         PID    PPID    PGID     SID TTY       COMMAND
heth      778254       1  778254  778254 ?        my_daemon
$ tail -f /var/log/sysg
Dec 20 12:37:59 emb3 my_daemon: Countet to 2
Dec 20 12:38:01 emb3 my_daemon: Countet to 3
Dec 20 12:38:03 emb3 my_daemon: Countet to 4
Dec 20 12:38:05 emb3 my_daemon: Countet to 5
...

Example of creating several processes

See same example in bash

 1 #include <stdio.h>
 2 #include <sys/types.h>
 3 #include <unistd.h>
 4 #include <wait.h>
 5 
 6 void childprocess(char *name, int count_to) {
 7         int counter = 0;
 8 
 9         printf("My name is %s and my PID is %d\n", name, getpid());
10 
11         while (counter < count_to) {
12                 sleep(1);
13                 counter = counter +1;
14                 printf("%s counted to %i\n", name, counter);
15         }
16         printf("%s signing off\n", name);
17 }
18 
19 
20 int main(void) {
21         int wstatus; // Used by c wait function
22 
23         printf("I am the main function and my PID is %i\n", getpid());
24 
25         if (fork() == 0) { // fork returns 0 for child
26                 childprocess("Hans", 5);
27                 return(0);      // Child process finished
28         }
29 
30         if (fork() == 0) {
31                 childprocess("Ulla", 3);
32                 return(0);
33         }
34 
35         while(wait(&wstatus) > 0);
36         printf("Main signing off\n");
37         return(0);
38 }